Photolithography is one method used to produce integrated circuit parts for semiconductor chips. Photolithography consists of processing substrate wafers with stepper machines that have been loaded with photomasks. This processing fabricates hundreds of integrated circuit parts on each wafer. The microscopic detail of the circuits created on the wafers makes photolithography a very exacting manufacturing method. Just a small change in the setup of the equipment or in one of the operating variables can drastically alter the resulting wafers.
However, in the course of production, one may wish to change the process. For example, although a current process may be providing for yields with good critical dimension behavior, the depth of focus may be unacceptable. A new process, using a different illumination setup may be desired. Altering the illumination may improve the depth of focus. The alteration may, however, cause a decline in critical dimension behavior. Other behaviors may also be of concern, including those caused by the photoresist process, etch process, or mask process.
What is needed is a process matching method which would allow the photolithography process to be changed without introducing variances within the resulting wafer images. The wafer image from the new process should be the same as the initial wafer image. In such a system, the results of the primary process are matched to the results of a secondary process.
One useful tool for improving a wafer image is optical proximity effect correction ("OPC"). OPC is a misnomer since the term now encompasses not only optical proximity-effect correction, but also etch proximity effect correction and all other forms of corrections to proximity effects. In OPC, a correction is made to a photomask design to compensate for dimensional and positional inaccuracies resulting from optical or etch effects. It is well known that there are several ways to accomplish OPC. One way is by a rule-based system. In such a system, a pattern for a photomask is analyzed by a series of rules, which isolate and adjust the problematic edges. A wafer printed from the OPC-corrected pattern should result in a flat printed "critical dimension vs. environment" response based on proximity.
A model-based system is a second type of OPC. A model is a mathematical description of a system. In a photolithography system, a model can describe optical, optical through resist, or other system. Regardless of the type of system described, there are generally two types of models. A theoretical model relies on the pure science involved in the system. Often, a theoretical model will be used to describe optical systems. Otherwise an empirical model can be used. In an empirical model, copious measurements are compiled and are then used to generate a set of formulae describing the behavior of the system.
In OPC, first, a simulation is made by applying the photomask pattern to the model. The resulting value is a prediction on how the pattern will print on the wafer. The prediction is the intensity return value. If normalized to a scale of 1, an intensity return value of 0 indicates chrome and a value of 1 indicates quartz. After obtaining the resulting value, then the factor of change needed to compensate for the proximity effect is calculated. In general, a threshold is determined, which is the intensity that is required to print on the wafer. For positive tone resist, above threshold equals clear substrate, below threshold equals resist. Then the intensity return value from the simulation is subtracted from the threshold value to find the intensity difference. This difference is then used to find the physical offset necessary to correct the pattern. This OPC method is repeated at close intervals throughout the pattern, resulting in a corrected photomask pattern.